VPX shorts

[wbhammack]

I've recently purchased a RV-8 with the VPX system and I keep running into a circuit that is shorting out. I've looked for shorts, swapped out boxes, and sent a autopilot control panel back to Dynon with no success. Working with a couple of guys that have built their own RV-8s and use the VPX system we think I may be down to swapping the wire / pin on the VPX system to see if the problem lies in that specific pin. If that doesn't work I may be down to sending the VPX back to get checked but I'm waiting to talk to those guys tomorrow.

My question is, has anyone ever run into this type of issue and had luck swapping out the wire/pin location in the VPX system. This is specifically a trim circuit but it's not shorting when the trim is being used. The timing of the short varies but it's never when the trim is being used.

Thanks in advance,
Skip

 

[riseric]

Check with the Configurator that for this particular pin\circuit that the "current fault" is OFF.
When that is activated, when the circuit is closed, it checks if there's a (too) low AMP value, (as when the trim motor is idle) then it suspects that there's a problem and opens the circuit.
From the VPX installation manual:
Current Fault: The pin can be configured to detect a “current
fault” or open circuit. When a circuit with this feature enabled is
turned on and does not draw any current for 3 seconds, the circuit
is faulted. You can use this to detect burned out lights, faulty boost
pump, failed avionics fan, or inop heated pitot tube for example. The
minimum detectable current is about 100ma, so this should be used
on devices like lights, strobes and loads that draw at least 100ma of
current.

 

[Aussieflyer]

Altenator

Are you using a Plane Power Alternator? I, and several others had similar tripping of VPX circuits. Long and short of the story was the Plane Power Alternator was producing dirty power spikes which cannot be seen on the ammeter or voltmeter. Easily observable on an oscilloscope. The VPX identifies the high voltage spike and trips the circuit in a millisecond to protect your instruments as it is designed to do.

Suffice to say we no longer use a Plane Power Alternator and have never experienced a tripped circuit since.

Alan

 

[clam]

I want to add a little color here as I’m trying to help troubleshoot this…
The offending circuit is the Dynon autotrim circuit (I don’t fly Dynon equipment, so I’m learning). Logic and power pass through the Dynon AP control head, so that was the reason the AP controller went back through Dynon for checkout.

Re: low current scenario: It isn’t a low current draw problem as we can observe the amps increase rapidly just prior to the circuit faulting. The current will sit at 0.3-ish up to about 2 amps, and all is well. Then the amps will rise rapidly to the mid-20’s and then fault.

Re: alternator issue: we don’t believe it’s an alternator issue (although it is a Plane Power) because this is the only circuit experiencing this anomaly.

This issue occurs 2-3 times every flight/ground run, but almost never at the same time. Actually, I don’t think it has happened in flight, but rather before or after during ground ops. The autotrim works as advertised during flight. We have thoroughly tested this with significant level speed changes and altitude changes where the autotrim gets a good workout. It has occurred with the alternator off as well.

 

[lr172]

I

Re: low current scenario: It isn’t a low current draw problem as we can observe the amps increase rapidly just prior to the circuit faulting. The current will sit at 0.3-ish up to about 2 amps, and all is well. Then the amps will rise rapidly to the mid-20’s and then fault.

a ray allen trim motor should only draw up to about .8 or 1 amp IIRC. So, just seeing it go up to 2 amps indicates there is an issue with that motor. Suggest you pull the wires from the servo motor off the autotrim unit and connect the trim motor to a battery through an ammeter and give it a work out that way. You can find a spec for max current draw somewhere on the web. The autotrim is likely just a relay, though could be a PWM driver. Either way, it is not likely to be the cullprit for the excessive current with the symptoms stated.

 

[riseric]

I originally connected my trim motor (standard Ray Allen supplied by Van's) through the VPX as per their installation manual.
But when I started to connect all the Dynon Skyview stuff, including the SV-AP-PANEL, I realised that if I wanted AUTO-TRIM when the Autopilot is engaged, all trim wiring (UP/DOWN and MOTOR) had to go through SV-AP-PANEL and nothing in VPX except for a 12V wire to the SV-AP-PANEL.
Just to understand better, is that circuit, feeding 12V to the SV-AP-PANEL, the one tripping?
What rating (Amps) is it set to?

Or is it a circuit powering something else??

 

[clam]

...trim wiring (UP/DOWN and MOTOR) had to go through SV-AP-PANEL and nothing in VPX except for a 12V wire to the SV-AP-PANEL.
Just to understand better, is that circuit, feeding 12V to the SV-AP-PANEL, the one tripping?
What rating (Amps) is it set to?

Or is it a circuit powering something else??

Owner can correct me if I state something incorrectly, as I am just a helper here, but yes, only the 12v power circuit is faulting. It was set to 8 amps. I believe he set it higher to 15 amps (not my advice) and got same results with circuit faulting.

"Possibly" complicating matters is that a toggle switch was added to the system to allow autopilot servo power and operation from the VPX in the event that the SV-AP panel failed and pilot still wanted autopilot functionality.

Not sure how common this power switching capability is? Or if it's recommended by Dynon? I have GRT equipment.

This airplane has operated in this configuration for over 100 hours trouble-free. So whatever is causing this didn't happen at installation. I'm leaning toward a loose crimp or bent wire or similar.

Or a bad VPX circuit, which was the question of OP.

 

[wbhammack]

What Clam said is correct. I met the previous owner/ builder yesterday at the hangar and he suggested we change it to 15 from 8 to see if that changed anything. It didn’t so I will reset it. The problem never happens when the trim is being used. I’ve gone from 100 knots to 170 knots with the autopilot on and it trims perfectly. Clam and I have discussed the possibility of a bad crimp etc or possibly something related to the VPX. I think he’s probably right about the wire problem but of course it would be easier to find if it was just flat out broken.

 

[riseric]

only the 12v power circuit is faulting. It was set to 8 amps. I believe he set it higher to 15 amps (not my advice) and got same results with circuit faulting.

"Possibly" complicating matters is that a toggle switch was added to the system to allow autopilot servo power and operation from the VPX in the event that the SV-AP panel failed and pilot still wanted autopilot functionality.

Not sure how common this power switching capability is? Or if it's recommended by Dynon? I have GRT equipment.

This airplane has operated in this configuration for over 100 hours trouble-free. So whatever is causing this didn't happen at installation. I'm leaning toward a loose crimp or bent wire or similar.

Or a bad VPX circuit, which was the question of OP.

Dynon installation manual state 4A for the 12V power feed to the AP-PANEL
I never heard nor read about a switch permitting a second power feed to the AP. Most definitely an additional risk of failure. Don't believe it's recommended by Dynon.

I might be mistaken, but in case of a AP-PANEL failure, the AP functions could still be available directly on the Skyview Screen ? The AUTO-TRIM might be inop though.
I would checkthe wire and crimps, from the VPX to the AP-PANEL for continuity. Replace the wiring ??
Check the pins of the D15 connector, specially # 2, 7, 8, 9, ?

Check the trim motor wiring and ground ?

I would certainly remove the extra switch and wiring.

If that doesn't cure the issue, with a sound wire I would change the VPX pin location and reconfigure.
Good luck !!